Probe for testing a device under test

ABSTRACT

A probe measurement system for measuring the electrical characteristics of integrated circuits or other microelectronic devices at high frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/975,476, filed Oct. 19, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/888,957, filed Aug. 3, 2007, which is acontinuation of U.S. patent application Ser. No. 11/391,895, filed Mar.28, 2006, now U.S. Pat. No. 7,271,603, which is a continuation of U.S.patent application Ser. No. 10/445,174, filed May 23, 2003, now U.S.Pat. No. 7,057,404.

BACKGROUND OF THE INVENTION

The present invention relates to probe measurement systems for measuringthe electrical characteristics of integrated circuits or othermicroelectronic devices at high frequencies.

There are many types of probing assemblies that have been developed forthe measurement of integrated circuits and other forms ofmicroelectronic devices. One representative type of assembly uses acircuit card on which the upper side are formed elongate conductivetraces that serve as signal and ground lines. A central opening isformed in the card, and a needle-like probe tip is attached to the endof each signal trace adjacent the opening so that a radially extendingarray of downwardly converging needle-like tips is presented by theassembly for selective connection with the closely spaced pads of themicroelectronic device being tested. A probe assembly of this type isshown, for example, in Harmon U.S. Pat. No. 3,445,770. This type ofprobing assembly, however, is unsuitable for use at higher frequencies,including microwave frequencies in the gigahertz range, because at suchfrequencies the needle-like tips act as inductive elements and becausethere are no adjoining elements present to suitably counteract thisinductance with a capacitive effect in a manner that would create abroadband characteristic of more or less resistive effect. Accordingly,a probing assembly of the type just described is unsuitable for use atmicrowave frequencies due to the high levels of signal reflection andsubstantial inductive losses that occur at the needle-like probe tips.

In order to obtain device measurements at somewhat higher frequenciesthan are possible with the basic probe card system described above,various related probing systems have been developed. Such systems areshown, for example, in Evans U.S. Pat. No. 3,849,728; Kikuchi JapanesePublication No. 1-209,380; Sang et al. U.S. Pat. No. 4,749,942; Lao etal. U.S. Pat. No. 4,593,243; and Shahriary U.S. Pat. No. 4,727,319. Yetanother related system is shown in Kawanabe Japanese Publication No.60-223,138 which describes a probe assembly having needle-like tipswhere the tips extend from a coaxial cable-like structure instead of aprobe card. A common feature of each of these systems is that the lengthof the isolated portion of each needle-like probe tip is limited to theregion immediately surrounding the device-under-test in order tominimize the region of discontinuity and the amount of inductive loss.However, this approach has resulted in only limited improvement inhigher frequency performance due to various practical limitations in theconstruction of these types of probes. In Lao et al., for example, thelength of each needle-like tip is minimized by using a wide conductiveblade to span the distance between each tip and the supporting probecard, and these blades, in turn, are designed to be arranged relative toeach other so as to form transmission line structures of stripline type.As a practical matter, however, it is difficult to join the thinvertical edge of each blade to the corresponding trace on the card whilemaintaining precisely the appropriate amount of face-to-face spacingbetween the blades and precisely the correct pitch between the ends ofthe needle-like probe tips.

One type of probing assembly that is capable of providing acontrolled-impedance low-loss path between its input terminal and theprobe tips is shown in Lockwood et al. U.S. Pat. No. 4,697,143. InLockwood et al., a ground-signal-ground arrangement of strip-likeconductive traces is formed on the underside of an alumina substrate soas to form a coplanar transmission line on the substrate. At one end,each associated pair of ground traces and the corresponding interposedsignal trace are connected to the outer conductor and the centerconductor, respectively, of a coaxial cable connector. At the other endof these traces, areas of wear-resistant conductive material areprovided in order to reliably establish electrical connection with therespective pads of the device to be tested. Layers of ferrite-containingmicrowave absorbing material are mounted about the substrate to absorbspurious microwave energy over a major portion of the length of eachground-signal-ground trace pattern. In accordance with this type ofconstruction, a controlled high-frequency impedance (e.g., 50 ohms) canbe presented at the probe tips to the device under test, and broadbandsignals that are within the range, for example, of DC to 18 gigahertzcan travel with little loss from one end of the probe assembly toanother along the coplanar transmission line formed by eachground-signal-ground trace pattern. The probing assembly shown inLockwood et al. fails to provide satisfactory electrical performance athigher microwave frequencies and there is a need in microwave probingtechnology for compliance to adjust for uneven probing pads.

To achieve improved spatial conformance between the tip conductors of aprobe and an array of non-planar device pads or surfaces, severalhigh-frequency probing assemblies have been developed. Such assembliesare described, for example, in Drake et al. U.S. Pat. No. 4,894,612;Coberly et al. U.S. Pat. No. 4,116,523; and Boll et al. U.S. Pat. No.4,871,964. The Drake et al. probing assembly includes a substrate on theunderside of which are formed a plurality of conductive traces whichcollectively form a coplanar transmission line. However, in oneembodiment shown in Drake et al., the tip end of the substrate isnotched so that each trace extends to the end of a separate tooth andthe substrate is made of moderately flexible nonceramic material. Themoderately flexible substrate permits, at least to a limited extent,independent flexure of each tooth relative to the other teeth so as toenable spatial conformance of the trace ends to slightly non-planarcontact surfaces on a device-under-test. However, the Drake et al.probing assembly has insufficient performance at high frequencies.

With respect to the probing assembly shown in Boll et al., as citedabove, the ground conductors comprise a pair of leaf-spring members therear portions of which are received into diametrically opposite slotsformed on the end of a miniature coaxial cable for electrical connectionwith the cylindrical outer conductor of that cable. The center conductorof the cable is extended beyond the end of the cable (i.e., as definedby the ends of the outer conductor and the inner dielectric) and isgradually tapered to form a pin-like member having a rounded point. Inaccordance with this construction, the pin-like extension of the centerconductor is disposed in spaced apart generally centered positionbetween the respective forward portions of the leaf-spring members andthereby forms, in combination with these leaf-spring members, a roughapproximation to a ground-signal-ground coplanar transmission linestructure. The advantage of this particular construction is that thepin-like extension of the cable's center conductor and the respectiveforward portions of the leaf-spring members are each movableindependently of each other so that the ends of these respective membersare able to establish spatially conforming contact with any non-planarcontact areas on a device being tested. On the other hand, thetransverse-spacing between the pin-like member and the respectiveleaf-spring members will vary depending on how forcefully the ends ofthese members are urged against the contact pads of thedevice-under-test. In other words, the transmission characteristic ofthis probing structure, which is dependent on the spacing between therespective tip members, will vary in an ill-defined manner during eachprobing cycle, especially at high microwave frequencies.

Burr et al., U.S. Pat. No. 5,565,788, disclose a microwave probe thatincludes a supporting section of a coaxial cable including an innerconductor coaxially surrounded by an outer conductor. A tip section ofthe microwave probe includes a central signal conductor and one or moreground conductors generally arranged normally in parallel relationshipto each other along a common plane with the central signal conductor soas to form a controlled impedance structure. The signal conductor iselectrically connected to the inner conductor and the ground conductorsare electrically connected to the outer conductor, as shown in FIG. 1. Ashield member is interconnected to the ground conductors and covers atleast a portion of the signal conductor on the bottom side of the tipsection. The shield member is tapered toward the tips with an openingfor the tips of the conductive fingers. The signal conductor and theground conductors each have an end portion extending beyond the shieldmember and the end portions are able to resiliently flex, despite thepresence of the shielding member, relative to each other and away fromtheir common plane so as to permit probing devices having non-planarsurfaces.

In another embodiment, Burr et al. disclose a microwave probe thatincludes a supporting section of a coaxial cable including an innerconductor coaxially surrounded by an outer conductor, as shown in FIGS.2A, 2B, and 2C. A tip section of the microwave probe includes a signalline extending along the top side of a dielectric substrate connecting aprobe finger with the inner conductor. A metallic shield may be affixedto the underside of the dielectric substrate and is electrically coupledto the outer metallic conductor. Ground-connected fingers are placedadjacent the signal line conductors and are connected to the metallicshield by way of vias through the dielectric substrate. The signalconductor is electrically connected to the inner conductor and theground plane is electrically connected to the outer conductor. Thesignal conductor and the ground conductor fingers (connected to theshield via vias) each have an end portion extending beyond the shieldmember and the end portions are able to resiliently flex, despite thepresence of the shielding member, relative to each other and away fromtheir common plane so as to permit probing devices having non-planarsurfaces. While the structures disclosed by Burr et al. are intended toprovide uniform results of a wide frequency range, they unfortunatelytend to have non-uniform response characteristics at high microwavefrequencies.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates an existing probe.

FIGS. 2A-2C illustrate another existing probe.

FIG. 3 illustrates one embodiment of a probe.

FIG. 4 illustrates a side view of a portion of the probe of FIG. 3.

FIG. 5 illustrates a bottom view of a portion of the probe of FIG. 3.

FIG. 6 illustrates another embodiment of a probe.

FIG. 7 illustrates yet another embodiment of a probe.

FIG. 8 illustrates still another embodiment of a probe.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present inventors considered the co-planar fingered probing devicesdisclosed by Burr et al., including the co-planar finger configurationand the microstrip configuration with fingers extending therefrom. Inboth cases, electromagnetic fields are created during probing betweenthe fingers. These electromagnetic fields encircle each of the fingers,electrically couple the signal finger to the ground fingers, andelectrically couple the ground fingers one another. While the probingdevice is being used for probing, the resulting electromagnetic fieldssurrounding the fingers interact with the wafer environment. Whileprobing in different regions of the wafer, the interaction between theelectromagnetic fields around the fingers and the wafer change,typically in an unknown manner. With a significant unknown change in theinteraction it is difficult, if not impossible, to accurately calibrateout the environmental conditions while probing a device under test.

When multiple probes are being simultaneously used for probing the samearea of the wafer, the probe tips come into close proximity with oneanother and result in additional coupling between the probes, normallyreferred to as cross-talk. In addition, the region between the supportfor the fingers, such as a dielectric substrate, and the extendedportion of the fingers results in a significant capacitance, whichimpedes high frequency measurements.

The present inventors were surprised to determine that the microstripstructure disclosed by Burr et al. further does not calibrate well oncalibration test substrates at very high frequencies, such as in excessof 70 GHz. This calibration is independent of potential interaction witha wafer at a later time during actual probing of a device under test.After examination of this unexpected non-calibration effect the presentinventors speculate that an energy is created in an “undesired mode”,other than the dominant field modes, at such extreme frequencies. This“undesired mode” results in unexpected current leakages from the signalpath thus degrading the signal integrity. The present inventors furtherspeculate that this “undesired mode” involves resonating energy in theground plane as a result of discontinuities in the ground path,including for example, the connection between the ground plane and theexternal portion of the cable, and the inductance in the ground plane.This ground plane resonant energy results in unpredictable changing ofthe energy in the signal path to the device under test, thus degradingperformance. This degradation wasn't apparent at lower operatingfrequencies, so accordingly, there was no motivation to modify existingprobe designs in order to eliminate or otherwise reduce its effects.

Referring to FIG. 3, a semi-rigid coaxial cable 40 is electricallyconnected at its rearward end to a connector (not shown). The coaxialcable 40 normally includes an inner conductor 41, a dielectric material42, and an outer conductor 43. The coaxial cable 40 may likewise includeother layers of materials, as desired. The forward end of the cable 40preferably remains freely suspended and, in this condition, serves as amovable support for the probing end of the probe.

A microstrip style probe tip 80 includes a dielectric substrate 88 thatis affixed to the end of the coaxial cable 40. The underside of thecable 40 is cut away to form a shelf 85, and the dielectric substrate 88is affixed to the shelf 85. Alternatively, the dielectric substrate 88may be supported by an upwardly facing shelf cut away from the cable orthe end of the cable without a shelf. Referring also to FIG. 4, aconductive shield 90, which is preferably planar in nature, is affixedto the bottom of the substrate 88. The conductive shield 90, may be forexample, a thin conductive material (or otherwise) that is affixed tothe substrate 88. By using a generally planar conductive material havinga low profile the shield 90 is less likely to interfere with the abilityto effectively probe a device under test by accidently contacting thedevice under test. The conductive shield 90 is electrically coupled tothe outer conductor 43 to form a ground plane. The other conductor 43 istypically connected to the ground, though the outer conductor 43 may beprovided with any suitable voltage potential (either DC or AC). Theconductive shield 90 preferably covers all of the lower surface of thesubstrate 88. Alternatively, the conductive shield 90 may cover greaterthan 50%, 60%, 70%, 80%, 90%, and/or the region directly under amajority (or more) of the length of a conductive signal trace on theopposing side of the substrate 88.

One or more conductive signal traces 92 are supported by the uppersurface of the substrate 88. The conductive traces 92, may be forexample, deposited using any technique or otherwise supported by theupper surface of the substrate. The conductive trace(s) 92 iselectrically interconnected to the inner conductor 41 of the coaxialcable 40. The inner conductor 41 of the coaxial cable 40 and theconductive trace(s) 92 normally carries the signal to and from thedevice under test. The conductive trace(s) 92 together with the shieldlayer 90 separated by a dielectric material 88 form one type of amicrostrip transmission structure. Other layers above, below, and/orbetween the shield layer 90 and the conductive trace 92 may be included,if desired.

To reduce the effects of the aforementioned unexpected high frequencysignal degradation, the present inventors determined that the signalpath may include a conductive via 94 passing through the substrate 88.The conductive via 94 provides a manner of transferring the signal pathfrom the upper surface of the substrate to the lower surface of thesubstrate. The conductive via 94 avoids the need for using a conductivefinger extending out from the end of the substrate 88 that wouldotherwise result in a significant capacitance between the extendedfinger and the end of the substrate 88. The conductive via 94 provides apath from one side of the substrate 88 to the other side of thesubstrate 88 in a manner free from an air gap between the conductive via94 and the substrate 88 for at least a majority of the thickness of thesubstrate 88. In addition, the shield layer 90 preferably extends beyondthe via 94 to provide additional shielding.

Referring also to FIG. 5, the lower surface of the substrate 88illustrates a contact bump 100 electrically connected to the via 94 andthe trace 92 extending below the lower surface of the substrate 88 andthe shield 90 which may be used to make contact with the device undertest during probing. The conductive shield 90 may include an “patterned”section around the contact “bump” 100 so that the shield layer 90 andthe signal path are free from being electrically interconnected (e.g.,the shield layer 90 may be greater than 50%, 75%, or laterallysurrounding all of the contact at some point). It is to be understoodthat the contact may take any suitable form, such as for example a bump,a patterned structure, a conductive structure, a needle structure, or anelongate conductor. The conductive shield 90 may laterally encircle theconductive bump which increases the resistance to externalelectromagnetic fields. Also, the conductive shield 90 extending beyondthe conductive bump 100 reduces the crosstalk from other probes. Forsome probing applications, one or more shield 90 contacts 102 may beprovided, if desired. The shield layer and the conductive trace arenormally constructed to provide a microstrip transmission linecontrolled impedance structure. While typically the signal line has atest signal and the shield has a ground potential, the two conductivepaths may likewise be any other configuration, such as balanced inputswhich vary with respect to ground.

Referring to FIG. 6, the probe may employ an outer cone shaped uppershield 10. The outer conductor 43 of the coaxial cable is connected tothe upper shield 110 is therefore electrically connected to ground. Thisdesign provides a smooth transition between the coaxial cable and theend of the probe. The probe is therefore shielded as it transitions tothe tip of the cone portion.

The upper shield 10 has a tapered cylindrical portion whose forward endis a tapered tip and whose rear end has a contour that is in continuouscontact with the outer coaxial conductor along its circumference so thatthere is no gap between the outer conductor and portions of the shieldthat could create fringing fields that could effect probe measurements.Likewise, any other shape may be used for the shield 110, as desired. Inaddition, the forward end preferably extends past the via and forms asubstantially closed region so that there is reduced fringing fields atthe forward end. The shield reduces parasitic coupling to any externalstructure and the construction of the shield as a single piece of metalreduces complexity of assembly. The shield is preferably made of a thinfoil and is capable of being formed by a fabrication process. The shieldmay also be deposited or made of other material.

The lower shield member 90 extends underneath the conductive tracebetween the fingers and the chuck holding the device under test. Theshield therefore helps to block the generation of ground plane resonantmodes that can otherwise interfere with and degrade the signal from thedevice under test.

Referring to FIG. 7, in an alternative embodiment a conductive finger112 or other elongate conductive element may be provided that iselectrically interconnected to the via. One or more additional groundfingers 114 may be electrically connected to the lower shield material.If desired, each respective finger may include a cantilevered portionthat extends down away from the substrate. The cantilevered portions arepreferably arranged in transversely spaced apart relationship to eachother so as to cooperatively form a controlled impedance transmissionline in order that a low loss transition can be made between therespective conductors on the cable and the respective pads on thedevice-under test.

While the use of an upper shield 110 that includes a curved surfaceprovides an improvement to signal integrity, the changes in thestructure of the upper shield tend to introduce some limitations intothe signal integrity at high frequencies, thus impeding performance. Forexample, the changes in the height of the upper shield changes theelectromagnetic field pattern along the length of the signal conductor.In addition, increased manufacturing complexity occurs with the uppershield. Furthermore, in most cases microwave microstrip transmissionstructures are enclosed in a housing, such as a conductive case, andaccordingly there is reduced motivation to include an upper shieldstructure.

To further increase the performance at high frequencies the presentinventors considered the effects of the substrate material. In manycases the dielectric constant of the dielectric substrate material ishigh, such as Al.sub.2O.sub.3 which has a 9.9 dielectric constant.Materials with a high dielectric constant have a tendency to concentratethe electromagnetic fields therein, thus decreasing the electromagneticfields susceptible to influence by other devices. In addition, thethickness of the substrate is typically 250-500 microns to providemechanical stability. Thus the fields tend to concentrate within thesubstrate.

Referring to FIG. 8, while considering such substrates the presentinventors came to the realization that the flexible membrane substratemay be substituted for the more rigid substrate 88. An example ofmembrane material is described in U.S. Pat. No. 5,914,613, incorporatedby reference herein together will all other references cited hereinincorporated by reference herein. In general, membrane based probes arecharacterized by a flexible (or semi-flexible) substrate with tracessupported thereon together with contacting portions being supportedthereon. The membrane portion of the probe may be constructed from asacrificial substrate into which is created a depression. Into thisdepression is located conductive material, traces are located thereon ifdesired, and flexible dielectric material is located on or under thetraces. Thereafter, the sacrificial substrate is removed leaving theprobe tip, traces, and membrane material. The contacting portions comeinto contact with the device under test and the traces are normally onthe opposing side of the membrane connected to the contacting portionsusing vias. In many cases, the membrane technology may be significantlythinner than ceramic based substrates, (see, e.g., substrate 88) such as40, 30, 20, 10, 5, or 3 microns or less. Normally the dielectricconstant of the membrane material is 7 or less, sometimes less than 6,5, or 4 depending on the particular material used. While normally usinga lower dielectric constant substrate is unsuitable, using asignificantly thinner substrate together with a lower dielectricconstant substrate raises the theoretical frequency range of effectivesignal transmission to 100's of GHz. The significantly thinner substratematerial permits positioning the lower shield material significantlycloser to the signal traces than the relatively thick ceramic substrate,and therefore tends to more tightly confine the electromagnetic fieldsthere between. With a tight confinement of the electric fields in themembrane material, the present inventors determined that the highfrequency performance of the membrane material may be increased bylocating an upper shield material above the membrane material. Moreover,the upper shield material should likewise be correspondingly close tothe signal path, so the curved upper shield material positioned at asignificant distance from the signal trace previously used may notnormally be sufficient. Accordingly, the shield material should bepatterned on the top of the membrane material with a dielectric betweenthe signal trace and the upper shield material. In many cases, thedistance between the signal trace and the upper shield directly abovethe signal trace should be no more than 10 times the distance betweenthe signal trace and the lower shield material. More preferably, theaforementioned distance would be preferably 7, 5, 4, or 2 times.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A probe comprising: (a) a dielectric substrate having a pair ofopposing major surfaces; (b) an elongate conductor suitable to beelectrically interconnected to a test signal supported by saidsubstrate; (c) a conductive member suitable to be electricallyinterconnected to a ground signal supported by said substrate whereinsaid conductive member together with said elongate conductor form acontrolled impedance transmission structure; (d) a conductive pathbetween a first side of said substrate and a second side of saidsubstrate in a manner free from an air gap between the conductive pathand an edge of said substrate for at least a majority of the thicknessof said substrate and said conductive path free from electricalinterconnection with said conductive member; and (e) a contactelectrically interconnected to said conductive path for testing a deviceunder test.
 2. The probe of claim 1 wherein said controlled impedancetransmission structure is microstrip.
 3. The probe of claim 1 whereinsaid edge is a via through said substrate.
 4. The probe of claim 1wherein said edge is an exterior peripheral edge of said substrate. 5.The probe of claim 1 wherein said elongate conductor is electricallyinterconnected to a central conductor of a coaxial cable.
 6. The probeof claim 5 wherein said conductive member is electrically connected to aconductor surrounding said central conductor of said coaxial cable. 7.The probe of claim 6 wherein said substrate is supported by said coaxialcable.
 8. The probe of claim 7 wherein said substrate is supported by ashelf of said coaxial cable.
 9. The probe of claim 1 wherein saidconductive member is substantially planar.
 10. The probe of claim 1wherein said dielectric substrate is semi-flexible.
 11. The probe ofclaim 10 wherein said dielectric substrate is a membrane.
 12. The probeof claim 1 wherein said dielectric substrate has a dielectric constantless than
 7. 13. The probe of claim 1 wherein said dielectric substratehas a dielectric constant less than
 5. 14. The probe of claim 1 whereinsaid dielectric substrate has a dielectric constant less than
 4. 15. Theprobe of claim 1 wherein said dielectric substrate has a dielectricconstant less than
 2. 16. The probe of claim 1 wherein said groundsignal is zero volts.
 17. The probe of claim 1 wherein said conductivemember covers greater than 50% of said second side of said substrate.18. The probe of claim 1 wherein said conductive member covers greaterthan 60% of said second side of said substrate.
 19. The probe of claim 1wherein said conductive member covers greater than 70% of said secondside of said substrate.
 20. The probe of claim 1 wherein said conductivemember covers greater than 80% of said second side of said substrate.21. The probe of claim 1 wherein said conductive member covers greaterthan 90% of said second side of said substrate.
 22. The probe of claim 1wherein said conductive member, said elongate conductor, and saidsubstrate collectively form a microstrip transmission structure.
 23. Theprobe of claim 1 wherein said conductive member laterally surrounds atleast 50% of said contact.
 24. The probe of claim 1 wherein saidconductive member laterally surrounds at least 75% of said contact. 25.The probe of claim 1 wherein said conductive member laterally surroundsat least 100% of said contact.
 26. The probe of claim 1 wherein saidcontact is in the form of a bump.
 27. The probe of claim 1 wherein saidsubstrate has a thickness less than 40 microns.
 28. The probe of claim 1wherein said substrate has a thickness less than 30 microns.
 29. Theprobe of claim 1 wherein said substrate has a thickness less than 20microns.